Heretofore the discrimination between wanted and unwanted returns in beacon interrogator-transponder and IFF (identification friend and foe) systems has been one of major concern. In either the air traffic identification and control situation or in the combat situation, interrogation pulses are directed towards one particular aircraft to which its transponder will respond to obtain a reliable identification. This wanted aircraft will be in the particular sector of interest and may be at a maximum range. Other aircraft at closer ranges in the side or rear lobes of the radiation pattern of the interrogator antenna may respond which can produce ambiguities and actually produce a false return.
The most general approach has been to improve the directivity of antennas and to suppress their side and rear lobes. Within the present state of the antenna art, the complete suppression of unwanted returns employing improved antenna directivity alone is not economically practical. There have been attempts to solve the problem via electronic means in the interrogator-receiver. A principal example of such electronic systems is disclosed in U.S. Pat. No. 2,962,713 to K. E. Harris et al in which the interrogator-receiver is gated and a complex dual interlaced trains of interrogation pulses are used. The interrogation pulses are of different level. Discrimination is obtained on the basis of return from the higher intensity pulse train as distinguished from the low intensity train.
Another approach to electronic discrimination is disclosed in U.S. Pat. No. 2,594,916 to H. D. Gulnac. This patent discloses an automatic gain control circuit which serves to control the level of the receivers as a function of the azimuth sweep. Another approach to side lobe suppression in interrogator-transponder systems is disclosed in U.S. Pat. No. 2,966,675 to A. E. Smoll. In accordance with the teaching of this patent, a second antenna having a different radiation pattern than the interrogator-receiver antenna is used and reference pulses are transmitted via the second antenna. Discrimination against side lobe return is achieved by comparison of the reference pulses with the return from the primary interrogation pulses. U.S. Pat. No. 3,122,737 to M. Setrin and 2,741,759 to C. V. Parker employ dual antennas at the interrogator and dual transmitters with discrimination between wanted and unwanted returns on the basis of the dissimilar antenna patterns and dissimilar pulse trains.
In conventional search, naval and some ground based radar systems, gain variation in the receiver known as sensitivity time control has been used in which the gain of the radar receiver is lowered immediately following transmission and increased steadily at the time when the sea return has disappeared. Such circuits are mentioned in the M.I.T. Radiation Laboratories Series, published in 1964 by Boston Technical Publishers, Inc., in Section 12-8 on page 460.